1. Field of the Invention
The present invention relates to devices, methods of operating the devices, and methods of manufacturing the devices. The present invention relates to a semiconductor device among the devices, and in particular, a substrate including a semiconductor (a semiconductor substrate), and in particular, a semiconductor substrate in which a semiconductor layer having single crystallinity or crystallinity similar to single crystallinity is bonded to a substrate having an insulating surface, such as glass. Further, the present invention relates to a semiconductor device which includes a circuit including a thin film transistor (hereinafter referred to as a TFT) formed using the semiconductor substrate. For example, the present invention relates to an electronic device on which an electro-optic device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element is mounted as components.
Note that in this specification, a semiconductor device refers to all devices which can function by utilizing semiconductor characteristics. An electro-optic device, a semiconductor circuit, and an electronic device are all included in the semiconductor device.
2. Description of the Related Art
A technology for forming a thin film transistor (a TFT) by using a semiconductor thin film (with a thickness of approximately several to several hundreds of nanometers) formed over a substrate having an insulating surface is widely applied to electronic devices such as an IC and an electro-optic device. In particular, a technology for forming a TFT with stable characteristics is important for an image display device including a plurality of pixels, each of which needs an active element.
When an image display device including a TFT is formed, a photolithography technique with high precision is necessary to obtain precise image display. Further, a substrate with a large area tends to be used in order to reduce manufacturing costs. In order to precisely form a TFT using such a large-area substrate, a large one-shot exposure device, a stepper exposure device, or the like is used.
Although a large area can be exposed to light at one time in a large one-shot exposure device, there is a problem in that variation in irradiation intensity and degree of parallelization of light is large. Accordingly, a stepper exposure device, in which such a problem is not likely to occur, is often used.
Note that there are limitations on a region which can be exposed to light at one time with a stepper exposure device. Accordingly, when the area larger than the region is exposed to light, exposure is repeated several times.
A semiconductor substrate called a silicon-on-insulator (an SOI substrate) which has a thin single-crystal semiconductor layer on an insulating layer has been developed instead of a silicon wafer which is manufactured by thinly slicing an ingot of a single-crystal semiconductor, and the SOI substrate is spreading as a substrate in manufacturing a microprocessor or the like. This is because an integrated circuit including the SOI substrate has a variety of characteristics necessary for higher performance of the integrated circuit, such as small power consumption in high-frequency operation due to small parasitic capacitance between a drain of a transistor and a substrate.
As a method for manufacturing an SOI substrate, a hydrogen ion implantation separation method is known (e.g., see Reference 1: U.S. Pat. No. 6,372,609). The hydrogen ion implantation separation method is a method by which hydrogen ions are implanted into a silicon wafer to form a microbubble layer at a predetermined depth from the surface, and ia thin silicon layer (an SOI layer) is bonded to another silicon wafer using the microbubble layer as a cleavage plane. In addition to heat treatment for separating the SOI layer, it is necessary to perform heat treatment in an oxidizing atmosphere to form an oxide film on the SOI layer, remove the oxide film, and perform heat treatment at 1000 to 1300° C. in a reducing atmosphere to increase bonding strength.
On the other hand, there is an attempt to form an SOI layer on an insulating substrate such as a glass substrate. As an example of an SOI substrate in which an SOI layer is formed on a glass substrate, an SOI substrate in which a thin single-crystal silicon layer is formed on a glass substrate having a coating film by a hydrogen ion implantation separation method is known (see Reference 2: U.S. Pat. No. 7,119,365). In this case also, a thin silicon layer (an SOI layer) is formed on the glass substrate in such a manner that a microbubble layer is formed at a predetermined depth from the surface by implantation of hydrogen ions to a piece of single-crystal silicon, the glass substrate and the piece of single-crystal silicon are bonded to each other, and thereafter, the piece of silicon is separated using the microbubble layer as a cleavage plane.